The F-T synthesis, invented by German chemists Fischer and Tropsch in 1923 as the core process in the Gas-To-Liquids (GTL) technology, originates from the preparation of synthetic fuel from syngas by coal gasification. The GTL process consists of the three major steps of (1) reforming of natural gas, (2) F-T synthesis from syngas and (3) reforming of F-T products. The F-T reaction is characterized by performing at a temperature of 200 to 350° C. and a pressure of 10 to 30 atm using iron or a cobalt-based catalyst.
For the F-T reaction, iron- and cobalt-based catalysts have been widely used. The iron-based catalysts were preferred in the past F-T reaction. Recently, however, cobalt-based catalysts are predominant to increase the production of liquid fuel or wax and to improve the conversion rate of carbon monoxide. Iron-based catalysts are characterized in that they are the most inexpensive F-T catalysts producing less methane at high temperature and having high selectivity for olefins, and their products can be utilized as source material in chemical industry as light olefin or -olefin, as well as fuel. In addition, a lot of byproducts, including alcohols, aldehydes, ketones and the like are produced with the concomitant production of hydrocarbons.
Cobalt-based catalysts are more expensive than iron-based catalysts. But, they have higher activity, longer lifetime and higher yield of liquid paraffinic hydrocarbon production with less CO2 formation. However, they can be used only at low temperature because a significant amount of CH4 is produced at high temperature. Further, with the usage of expensive cobalt, the catalysts must be prepared by dispersing on a stable support with a large surface area, such as alumina, silica, titania and the like. Further, there is a need to add a small amount of precious metal components such as Pt, Ru, Re, etc., as promoters.
At present, there are four types of F-T synthesis reactors developed to date: a circulating fluidized bed reactor, a fluidized bed reactor, a multi-tubular fixed bed reactor and a slurry-phase reactor. The reactor should be adequately selected considering the composition of syngas and the kind of a final product, because they have different reaction characteristics depending in the type of a reactor. The F-T reaction conditions are also determined by the kind of final products. Typically, the high-temperature F-T process for producing gasoline and olefin is carried out in the fluidized bed reactor, and the low-temperature F-T process for producing wax and lubricant base oil is carried out in the multi-tubular fixed bed reactor (MTFBR) or in the slurry-phase reactor. Mostly, linear-chain paraffins are produced by the F-T synthesis reaction, but CnH2n compounds having double bonds, -olefins or alcohols are obtained as byproducts.
The synthesis mechanism of straight-chain hydrocarbons as a main product is mainly explained by the Schulz-Flory polymerization kinetic model. In the F-T process, more than 60% of primary products have a boiling point higher than that of diesel oil. Thus, diesel oil can be further produced by the subsequent hydrocracking process, and the wax component can be transformed into a high-quality lubricant base oil through a dewaxing process.
Furthermore, the slurry reactor for F-T synthesis has several advantages over the fixed bed reactor, as follows:                it shows high heat transfer efficiency and low pressure drop and temperature gradient along the axial direction of the reactor;        it is possible to add, discharge and recycle of a catalyst during the operation;        its production and installation are economically favorable; and        it yields a higher amount of products per unit reactor volume.        
Due to the above advantages, it has been preferred to use the slurry reactor rather than the fixed bed reactor. However, the slurry reactor has the problem of requiring an additional filtration step capable of separating solid catalyst particles and liquid products. Further, there is a problem in that catalyst aggregation caused during the F-T reaction leads to the decrease in reaction efficiency of a slurry reactor and the clogging of a filter used in the above filtration step.
For the prevention of catalyst deactivation through the improvement of catalyst activity and easy transfer of products, there is a method of preparing a silica-alumina catalyst having a bimodal pore structure and improving the stability of the catalyst by increasing the transfer rate of hydrocarbon compounds having a high boiling point produced during the F-T reaction using the same [US 2005/0107479 A1; Applied Catalysis A 292 (2005) 252]. The F-T catalyst prepared by the above method show various specific surface areas and a dual porosity structure. However, it has been reported that the F-T reaction activity is closely related with the cobalt particle size, pore size distribution of a support, reduction degree of cobalt and catalyst dispersion in a slurry reactor. Thus, it is better to utilize a method of preventing catalyst aggregation and filter clogging by using by-products such as alcohol produced during the F-T process as a co-solvent in slurry phase reaction in accordance with the present invention than to utilize the above method of using a support prepared by a complicated process. The method according to the present invention can contrive a more efficient F-T process by preventing the decrease in catalyst activity and improving a yield of higher hydrocarbons such as C5+ hydrocarbons.
According to the previous reports to the effect of alcohol added during the F-T reaction, it has been found that C2-C3 alcohols are decomposed during the reaction and convert Co metal components into a cobalt oxidation (CoO) state while decreasing catalyst activity. Simultaneously, ethane produced by the above alcohol decomposition is helpful to the chain growth of hydrocarbons in the F-T reaction, and thereby, enhances the production of hydrocarbons having a low boiling point [Fuel 86 (2007) 73-80]. Further, it has been reported that in case of additionally adding 1-olefin to a slurry reactor, it is possible to produce hydrocarbons having a high boiling point due to the enhanced chain growth of hydrocarbons [Topics in Catalysts, 2 (1995) 259-266]. However, there is no guarantee that the catalyst aggregation caused by the addition of 1-olefin during the slurry phase reaction can be effectively prevented. In general, it has been regarded that the catalyst aggregation is a phenomenon caused by depositing hydrocarbons having a high boiling point produced during the F-T reaction on the catalyst surface. Thus, in case of producing wax having a high boiling point due to the addition of 1-olefin, it is of advantage to the synthesis of hydrocarbons having a high boiling point, but is not suitable for the prevention of catalyst aggregation. Further, it has been reported that wax generated in the mesopore pore structure of a catalyst during the reaction may decrease the catalyst activity due to low transfer rate by a capillary effect, however, a small amount of alcohol generated during the reaction is helpful to the emulsion formation of water and wax, and the enhanced diffusion of H2 and CO in a condensed water phase of the mesopore can increase the F-T reactivity [Applied Catalysis A 161 (1997) 59-78].
In addition, for the facile transfer of wax generated during the F-T reaction in an emulsion state through a pipeline, U.S. Pat. No. 6,284,806 B2 (2001) and U.S. Pat. No. 6,677,388 B2 (2004) disclose a method of preparing F-T wax components in an emulsion state by using a non-ionic surfactant and water generated during the F-T reaction and delivering the same. According to the above patents, in case of using a cobalt-based catalyst, the typical distribution rate of oxygen-containing compounds existed in water generated during the F-T reaction is as follows:
C1-C12 alcohols: 0.05-1.5 wt %
C2-C6 acids: 0-50 wppm
C2-C6 ketones, aldehydes, acetals: 0-50 wppm
Other oxygen-containing compounds: 0-500 wppm
Of the above described components, alcohols can coexist with water, oils and wax, and the oxygen-containing compounds generated by the F-T reaction are known to be about 2-3 wt. % based on total hydrocarbons production.